Some rocks beneath us came from far away

Rocks west of the Bear Creek Valley define the "Applegate Group" of the Siskiyous, a part of the larger Klamath Mountains geologic province.

By Jad D'Allura

Rocks west of the Bear Creek Valley define the "Applegate Group" of the Siskiyous, a part of the larger Klamath Mountains geologic province.

They are older and infinitely more complex than younger rocks east of and including the Bear Creek Valley. They were squeezed, folded and broken multiple times like dough pounded by a 300-pound chef with a grudge.

During that abuse, they were metamorphosed (changed into quite different rocks) under temperatures probably around 450 degrees Celsius (842 degrees F) and pressures equivalent to six to seven miles of burial. Few age dates exist, but the rocks are at least 173 million years old and, in some areas, likely older.

Because they've been so badly mauled, occur in steep terrain and are covered with green stuff (vegetation), they haven't attracted detailed study. Still, we know a little about them. The bulk of the rock consists of volcanic debris and ash along with a few lava flows, most deposited in a submarine environment.

Intriguingly, they weren't originally part of North America but were formed offshore like the Indonesian Island chain. These volcanic island chains mashed into other island chains during a number of "docking" episodes, then onto our continent about 150 million years ago.

If that violence wasn't enough, some were broken and slid like playing cards northward along San Andreas-type faults. One could envision those "suspect terranes," as geologists call such rocks with non-North America flora and fauna, as visitors to our region. A few ventured to British Columbia and Alaska, where they now reside.

Applegate Group rocks were metamorphosed, generating new minerals that were stable under changed conditions and reduced pore spaces.

That made the rocks tighter and harder. Many rocks break into angular fragments and have a slight greenish cast due to microscopic green minerals such as chlorite (tiny flat green flakes) and epidote (miniscule yellow-green knots). Some of the rock (chert) was hard enough to be fashioned as tools by local Native Americans.

Such rocks are exposed in the large road cut opposite the parking lot near Applegate Dam. There metamorphosed volcanic rocks are cut by a couple of dark bands that represent dikes (dark bands of magma) injected into cracks. The dikes are cut by faults that offset them. There are also rips in the rock ("tension gashes") healed with white quartz. Farther south along the road, shiny green serpentinite is found. The serpentinite, originally part of the Earth's subsurface mantle, was faulted into place during deformation. Juxtaposition of such dissimilar rocks attests to deformation so severe it would make Wile E. Coyote cringe.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.